Plasmodium falciparum: linkage disequilibrium between loci in chromosomes 7 and 5 and chloroquine selective pressure in Northern Nigeria.

In view of the recent discovery (Molecular Cell 6, 861-871) of a (Lys76Thr) codon change in gene pfcrt on chromosome 7 which determines in vitro chloroquine resistance in Plasmodium falciparum, we have re-examined samples taken before treatment in our study in Zaria, Northern Nigeria (Parasitology, 119, 343-348). Drug resistance was present in 5/5 cases where the pfcrt 76Thr codon change was seen (100% positive predictive value). Drug sensitivity was found in 26/28 cases where the change was absent (93% negative predictive value). Allele pfcrt 76Thr showed strong linkage disequilibrium with pfmdr1 Tyr86 on chromosome 5, more complete than that between pfcrt and cg2 alleles situated between recombination cross-over points on chromosome 7. Physical linkage of cg2 with pfcrt may account for linkage disequilibrium between their alleles but in the case of genes pfmdr1 and pfcrt, on different chromosomes, it is likely that this is maintained epistatically through the selective pressure of chloroquine

Can pretreatment screening for dhps and dhfr point mutations in Plasmodium falciparum infections be used to predict sulfadoxine-pyrimethamine treatment failure?

This study examines the relationship between malaria treatment failure after sulfadoxine-pyrimethamine (S-P) chemotherapy and presence of mutations in the Plasmodium falciparum dihydropteroate synthase (dhps) and dihydrofolate reductase (dhfr) genes (associated with resistance in vitro to S and P) before treatment. In Kenya, 38 malaria patients in a holoendemic area, and 21 in an epidemic area, participated in the trial in 1997-98. In the 2 areas, drug failure occurred in 76% and 75% of cases where any mutation in dhfr was seen (positive predictive values 76% and 75%: P = 0.003 and 0.008) and an identical association was seen with dhfr Asn-108. In the holoendemic area all occurrences of > or = 2 mutations in dhfr predicted drug failure. Only 3 instances were seen in the epidemic focus, but treatment failed in all. Only in the epidemic focus, 7 (88%) of 8 occurrences of > or = 1 mutations in dhps, and all occurrences of the Gly-437 allele of dhps, predicted failure. Association between mutations in dhps and mutations in dhfr was noted in the combined sites, irrespective of outcome. Although this makes the relationship of combined dhfr and dhps mutations to failure more difficult to interpret, it nevertheless supports S-P selection acting on both genes. In the holoendemic site, treatment success increased with age. In this location, acquired immunity may mask the impact of mutations in dhps, since sulfadoxine is a less effective treatment than pyrimethamine

Co-trimoxazole compared with sulfadoxine-pyrimethamine in the treatment of uncomplicated malaria in Kenyan children.

Sulfadoxine-pyrimethamine (SP) and co-trimoxazole were both effective in reducing fever, clearing parasitaemia and improving anaemia in children aged < 5 years with uncomplicated malaria in 2 Kenyan endemic sites, Oyugis in the west and Tiwi on the coast. We compared the efficacy of these 2 regimens (in May-July 1998) by evaluating clinical and parasitological responses over 14 days. The combined incidence of parasitological failure for the combined sites for co-trimoxazole was 14/123 (11%) and for SP 23/145 (16%) (RR 0.72, 95% confidence interval [CI] 0.31-1.46, P = 0.289). The 14-day clinical failure rate for the combined sites for co-trimoxazole was 4/123 (3.3%), and for SP 8/145 (5.5%), (RR 1.69, 95% CI 0.91-3.15, P = 0.129). The results indicate that the risk of treatment failure for the 2 regimens was similar. The antimalarial use of co-trimoxazole in uncomplicated malaria needs further investigation, since the 10-12-h elimination half-life of both components should reduce selective pressure for resistance. In addition, use of a 2-day high-dose course, tested previously, requires further study to demonstrate its efficacy

Plasmodium falciparum in Kenya: high prevalence of drug-resistance-associated polymorphisms in hospital admissions with severe malaria in an epidemic area.

During an epidemic of Plasmodium falciparum malaria in Chogoria, Kenya, P. falciparum DNA was collected from 24 cases of severe malaria admitted to hospital for parenteral quinine treatment. These patients had all failed first- (chloroquine) and second-line (sulfadoxine-pyrimethamine or amodiaquine) drug treatments. Twenty-two (92%) of the 24 patients sampled carried parasites with the (Asn)86(Tyr) point mutation in the pfmdr1 gene (chromosome 5), 20 (83%) had an (Asp)1246(Tyr) mutation and 18 (82%) had both of these mutations. These alleles are both reported to be associated with chloroquine-resistance. Polymorphisms in the cg2 gene (chromosome 7) are also associated with chloroquine resistance, and 18 (75%) of the 24 parasite samples each had the cg2 and pfmdr1 polymorphisms. These 18 samples also had the mutations associated with resistance to pyrimethamine and sulfadoxine: (Asn)51(Ile), (Cys)59(Arg) and (Ser)108(Asn) of gene dhfr (chromosome 4) and (Ala)437(Gly) and (Lys)540(Glu) of dhps (chromosome 8), respectively. Genotyping of the parasites from all 24 patients revealed extensive diversity in the sequences for the merozoite surface antigens (MSA-1 and MSA-2) and the glutamate-rich protein (GLURP) and indicated that each sample contained more than one parasite clone. Although samples from non-admitted malaria cases were not available, it appears that drug resistance may have played an important role in the development of severe malaria in this epidemic

Mode of action of artemether lumefantrine (COARTEM): The sole, fixed, oral ADCC and its role in combatting multidrug resistance

Lumefantrine binds to hemin produced during hemoglobin breakdown, preventing detoxification to crystalline malaria pigment (hemozoin). During the same process, the perOXide group in artemether binds to heme and releases toxic free-radicals. Resistance to both lumefantrine and artemether depends on expression of a multidrug-resistance protein PGH-1. This shared resistance mechanism is responsible for potentiation between the components of coartemether. Sensitivity to lumefantrine and artemether is determined by mutations in PGH-1 associated with chloroquine-resistance, but if mutated PGH-1 is over-expressed, resistance to all three drugs may be seen. In Africa and S.America, where PGH-1 is not generally found amplified, but chloroquine-resistance is widespread, resistance to either component of coartemether is expected to be rare. This feature should be maintained especially where chloroquine is still in regular use. In areas of S.E.Asia where chloroquine and mefloquine are widely used, both wild type and mutated PGH-1 are present and amplified. To combat the consequent low-level resistance to the combination, higher treatment doses of co-artemether are required. The advantage of co-artemether as opposed to artesunate/mefloquine is seen in the shorter half-life of lumefantrine, allowing less time for resistance to be selected, and in co-artemether's better tolerability. The selection of chloroquine-sensitivity determinants by co-artemether, and of co-artemethersensitivity determinants by chloroquine suggests a possible strategy for resistance prevention in Africa and S. America, which may be enhanced by probable effects of artemether on transmissibility of gametocytes

[Presence of the double pfmdr1 mutation 86Tyr and 1246 Tyr in clones of a chloroquine-resistant west African isolate of Plasmodium falciparum].

Isolates of Plasmodium falciparum from three areas of West Africa were recovered from cryopreservation and their chloroquine-sensitivity were determined in vitro. Of the 90 samples studied, 60 were from Guinea-Bissau (30Resistant/30Sensitive), 15 were from S. Tomé and Príncipe (11Resistant/4Sensitive) and 15 were from Angola (11Resistant/4Sensitive). All the isolates were sensitive to mefloquine. Using the polymerase chain reaction/restriction fragment length polymorphism technique (PCR/RFLP) it was possible to detect two mutations in the pfmdr1 gene, often associated with chloroquine-resistance. 66% of the samples from Guiné-Bissau showed a correlation with chloroquine-resistance while 73% of the samples from São Tomé and Angola altogether had the 86Tyr mutation. The present study on West African isolates and clones showed, for the first time, the presence of a double point mutation in the pfmdr1 gene one being found, up to now, only in South America isolates of Plasmodium falciparum